Field of the Invention
The present invention relates to a pretensioner of a seat belt for a vehicle. More particularly, it relates to a pretensioner of a seat belt for a vehicle, which can prevent locking-dip and overshoot and achieve reduction of performance deviation, improvement of operational reliability, and improvement of passenger protection performance as a restraint apparatus.
Description of Related Art
Generally, seat belts can secure passengers upon crash of vehicles and prevent passengers from being thrown forward to minimize their injuries. When there is a crash while passengers are wearing seat belts, the forward movement of passengers should be prevented by the seat belts. However, since seat belts are unwound to a certain degree until seat belts (webbing) are locked by a retractor due to an impact generated at the crash moment, passengers may not be appropriately protected.
Passengers are thrown forward by a great external force during the crash. Even though passengers are secured by seat belts, a certain portion of seat belt is unwound due to an elongation of a webbing and a time delay of a retractor lock, allowing the upper body of a passenger to move forward.
Accordingly, a pretensioner is being used to reduce the injury of passengers by instantaneously rewinding the seat belt (webbing) thus early restraining the movement of passengers upon crash of vehicles. When the seat belt (webbing) is instantaneously rewound, the tension of the seat belt increases, thereby minimizing the forward movement of a passenger and reducing the injury of a passenger.
Mechanisms for rewinding the webbing to instantaneously operate the pretensioner are typically divided into a rack & pinion type and a gear rotation type using a steel-ball.
FIG. 3 is a view illustrating a typical steel-ball type pretensioner. As shown in FIG. 3, steel-balls 3 are pushed by a piston 2 formed of silicon due to a gaseous pressure generated by explosion of a Micro Gas Generator (MGG) 1, and are engaged with a gear 4 to allow the gear 4 and a spool to be instantaneously rotated and thus a webbing to be wound (normal rotation)
However, in this steel-ball type pretensioner, since a tube 6 that is a gas passage is advantageous in package space because it is configured to be bent, a performance deviation may occur due to a clearance between steel-balls, a ball jam in the gear due to the clearance, a deformation and jam of the piston, an energy loss in transmission of an explosive power of the MGG 1, and residual gas pressure/friction.
For the actuation of a load limiter after the rewinding of the pretensioner (i.e., for torsion of a torsion bar), a separate locking unit is needed to restrain the rotation of the spool (rotation of the spool is restrained after slight reverse rotation)
In this case, when a rotation element in the pretensioner is not locked, the spool counter rotates, and thus the webbing is unwound.
When the reverse rotation (until the rotation of the spool is restrained) of the spool occurs by the operation of the pretensioner, a residual gas cannot escape. In this case, as shown in FIG. 4, since a resistance (residual pressure) of a residual gas occurs, the initial load of the belt may significantly increase in a moment (overshoot). When a vent hole is processed to prevent the overshot, the initial restraint force may be reduced.
Also, as shown in FIG. 5, when the spool counter rotates, the silicon piston 2 is deformed to be jammed between the ball 3 and the tube 6, causing occurrence of a resistance and increasing the initial load.
In addition, since all balls 3 are not integrally combined, as shown in FIG. 6, the balls 3 may be jammed in the gear 4 or a dislocation (phase difference) between the ball 3 and the gear 4 may occur, causing an overshoot in which the initial belt load increases.
FIG. 7 is a view illustrating a typical limitation, which shows a load applied to the seat belt by the pretensioner upon crash, and shows that an overshoot in which the initial belt load increases occurs in a typical pretensioner.
Due to the overshoot, a certain performance is difficult to achieve, and a performance deviation occurs. Thus, there is a difficulty in tuning of the specifications of the restraint apparatus and the injury of passengers may become severe upon crash.
FIG. 8 is a view illustrating a typical rack & pinion type pretensioner. In the rack & pinion type pretensioner, the linear motion of a piston 8 and a rack 9 causes a pinion 10 to rotate due to a gas pressure generated by the explosion of a MGG 7. When a spool receives a rotary force of the pinion 10 and rotates, a webbing is instantaneously wound around the spool.
However, although force transmission by the explosion of the MGG 7 is advantageous, the size of the rack & pinion type pretensioner is great and the rack 9 that linearly moves occupies much space. Accordingly, the rack & pinion type pretensioner has a limitation in that the package space increases and thus its weight and cost increases.
Also, in both steel-ball type and rack & pinion type, although a residual gaseous pressure or a separate locking unit (locking a rotation element in the pretensioner) is used to operate a load limiter, particularly, to fix the spool after the operation of the pretensioner (after rewinding of the seat belt webbing), these cause a variation in action and performance, and make it difficult to control and make it complicated to configure the apparatus.
Also, as shown in FIG. 7, locking-dip that is an instantaneous reduction of the belt load after the operation of the pretensioner may occur in the spool fixing type that uses a residual gaseous pressure.
The locking-dip is a phenomenon in which the load increases due to the gaseous pressure inside the tube after the operation of the pretensioner, and then the gaseous pressure decreases before an Emergency Locking Retractor (ELR) for fixing the seat belt webbing is locked and the load again increases after the ELR lock. It is known that the locking-dip occurs in most typical pretensioners to a greater or lesser extent.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.